Many electric machines, such as electric motors and electric generators, include a stator that is held stationary and a rotor that rotates adjacent the stator. The stator and rotor may be configured to transfer power between one another through one or more rotating magnetic fields. Some electric machines may include a permanent-magnet type rotor with permanent magnets mounted on or inside a rotor core of the rotor. Each permanent magnet of the rotor may individually create a north or south magnetic pole of the rotor. A permanent-magnet type rotor having only a single permanent magnet creating each of its magnetic poles may, however, limit the performance potential of the associated electric machine.
U.S. Pat. No. 6,664,688 to Naito et al. (“the '688 patent”) shows a rotor with each of its magnetic poles created by a group of permanent magnets. Each group of permanent magnets of the '688 patent includes an outer permanent magnet disposed in a recess in an outer surface of a rotor core. Additionally, each group of permanent magnets of the rotor disclosed by the '688 patent includes two arc-shaped inner permanent magnets mounted in cavities in the rotor core. Inner ends of the two inner permanent magnets are disposed adjacent one another, radially inward of the outer permanent magnet of the group. Outer ends of the two inner permanent magnets of each group are disposed at the outer surface of the rotor on opposite sides of the outer permanent magnet. A relatively thin portion of the rotor core between the inner ends of the two inner permanent magnets provides the only connection between a portion of the rotor core disposed radially outward of the two inner permanent magnets and other portions of the rotor core.
Although each of the magnetic poles of the rotor of the '688 patent is created by multiple permanent magnets, certain disadvantages persist. For example, the relatively narrow portion of the rotor core that extends between the inner ends of the two inner permanent magnets may be subjected to undesirably high stresses during rotation of the rotor. Rotation of the rotor may create centrifugal force on the portion of the rotor core disposed radially outward the inner permanent magnets. In order to keep the rotor core intact, the relatively thin portion of the rotor core between inner ends of the two inner permanent magnets must counteract all of the centrifugal force on the portion of the rotor core disposed radially outward of the two inner permanent magnets. Additionally, the shape of the outer permanent magnet and the cavity in which it is mounted may make the outer permanent magnet susceptible to detachment from the rotor core during high-speed rotation of the rotor. Furthermore, because they are arc-shaped, the two inner permanent magnets may be expensive.
The electric machine and rotor of the present disclosure solve one or more of the problems set forth above.